Waveguide
High-Speed Circuits and Systems Laboratory
B.M.Yu
High-Speed Circuits and Systems
Laboratory
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Content
1. Overview
2. Introduction
3. Design and fabrication
I.
Simulation
II.
measurement
4. Waveguide loss measurement
5. Coupling between shallow-ridge and narrow strip
6. Conclusion
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Laboratory
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Overview
Optics Express (2010), Low Loss Shallow-Ridge Silicon Waveguide, Po Dong
2 um
0.25 um
3 um
Cross section of WG
 Buried Oxide: 3 um, Cross section of waveguide: 0.25 um x 2 um
 Target : Chip to Chip interconnect (a few tens of centimeter)
 Average propagation loss: 0.274±0.008 dB/cm in C-band (1530~1565 nm)
 Double-level taper: to adiabatically couple from shallow ridge to strip waveguide
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Introduction
 Silicon photonics: interest area for broad spectrum applications (optical
interconnect, sensing, RF photonics)
 Submicron wide deeply etched waveguide structures
- Efficient and high speed active photonic devices
- SOI substrates (Top silicon thickness: 0.25 um)
- Lowest propagation loss (in previous reports): 1~2 dB/cm @ 1550 nm
- Chip to Chip interconnect & Narrow bandwidth filters in RF photonics
 Shallow ridge or thin silicon waveguide
- Propagation loss: 0.3~1 dB/cm (selective oxidation fabrication technique)
- Difficult to control (device density, hard mask thickness, cross section of
WG)
 In this paper
- Low loss silicon ridge WG: 0.25 um silicon, average propagation loss:
0.274 dB/cm
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Design and Fabrication
 Simulation
- Main reason of waveguide propagation loss: light scattering from etch
sidewalls
 Minimizing optical field overlap with etched interface (increasing width of WG,
decreasing etch depth)
- Cross section of wave guide: 2 um x 0.25 um (etch depth: 0.05 um)
- Power confinement: 84 %
Etch sidewall of WG
Shallow-ridge WG
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Design and Fabrication
 Simulation
Group index: ~3.7, effective index: ~2.9
- Etch depth tolerance ±0.01 um
- Group index variation: 0.0033  5ps delay time difference (50cm waveguide)
40Gbps with a reasonable fabrication tolerance
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Design and Fabrication
 Simulation
bending loss with various bending radii
 Bending loss
- 90° bending with various radii (50~120 um)
- @ 100um radii 10-4dB
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Design and Fabrication
 Fabrication
6 mm
3 mm
SEM image of WG cross section
Top-view of 64 cm waveguide
- Soitec 6” wafers
- 0.25 um thick silicon 3um buried oxide
- Spiral waveguide (rmin= 300 um)
- 6 mm x 3 mm waveguide (length of waveguide: 64cm)
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Waveguide loss measurement
 Test setup
Optical fiber (TE mode Polarization)
ASE (𝜆 =1550 nm)
Waveguide
(horizontal taper)
Optical fiber
Optical spectrum analyzer
High-Speed Circuits and Systems
Laboratory
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Waveguide loss measurement
 Loss measurement
Insertion loss measured for different
WG length as a function of wavelength
Waveguide propagation loss using
linear fitting @ 𝜆 =1550 nm
- Insertion loss is normalized to power measured from direct to
direct fiber coupling
- Insertion loss = waveguide propagation loss + coupling loss
- Waveguide loss: 0.281dB/cm (@ 𝜆 =1550 nm)
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Waveguide loss measurement
 Loss measurement
Waveguide propagation loss as a
function of wavelength in C-band
Waveguide loss with 9 chip on the
same 6” SOI wafer
- Average loss (𝜆 variation): 0.274 dB/cm ± 0.008 dB/cm
- Same wafer but different average loss  etch depth variation
- Average loss (same wafer): 0.299 dB/cm
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Coupling between shallow-ridge and narrow strip WG
 Double level taper
Coupling between shallow-ridge
and narrow strip waveguide
3-D simulation result
- Narrow strip WG (450 nm x 250 nm): 1.5 um bending radius
- In Ring modulator, narrow strip WG is more efficient.
- Coupling would be need
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Coupling between shallow-ridge and narrow strip WG
 Simulation result
Coupling loss as a function of taper length
- 10um long taper is sufficient in order to achieve <0.25 dB coupling loss
- Highly index contrast between silicon and oxide
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